Flash memory devices are nonvolatile semiconductor memory devices that are electrically erasable and programmable. In flash memory devices, multiple memory blocks may be written with data in a single programming operation. In general, in a flash memory device, one memory block is erasable or programmable at a time. This means that flash memory devices may more rapidly and/or effectively read and/or write data when systems employing such flash memory devices read and write data in blocks. Flash memory devices usually include insulation films that enclose charge storage elements used for storing data. These insulation films may become worn out after a certain number of read/write operations.
Flash memory devices may continue to store data even when power is not supplied to the chip. That is, flash memory devices may be able to retain stored information therein without consuming a substantial amount of power even after the power supply to the chips has been interrupted. In addition, flash memory devices may offer resistance to physical shocks and/or fast read access times. Due in part to such features, flash memory devices are widely used as storage units in battery powered electronic devices. Flash memory devices are generally classified as NOR or NAND-type devices depending on the logical arrangement of gates in the memory.
In a flash memory device, information is stored in an array of transistors, each of which acts as a unit cell storing one bit of information. Some types of advanced flash memory devices, called multi-level flash memory devices, are capable of storing more than one data bit in a unit cell by varying the amount of electric charge stored in a floating gate of cell.
A unit cell of a flash memory device may have a structure similar to a typical MOSFET, but with two gates. In particular, a flash memory cell typically includes a control gate, such as the gate included in a conventional MOSFET device. In addition, a flash memory cell may also include a floating gate enclosed by an insulation film and disposed between the control gate and the substrate. The floating gate is isolated from peripheral conductors by the insulation film, and electrons are captured in the floating gate to preserve information without leakage. Electrons captured in the floating gate change an electric field between the control gate and the substrate, which may vary a threshold voltage of the cell. Thus, in order to read a data bit from the cell, a specific voltage is applied to the control gate. In response to the gate voltage, a current may flow, or not flow, through the cell depending on the threshold voltage of the cell. Since the threshold voltage of the cell is affected by the amount of charge accumulated in the floating gate, it is possible to determine if a data 1 or data 0 is stored in the cell by detecting a flow of current through the cell in response to the gate voltage.
A flash memory cell may experience a lowering of the threshold voltage over time, possibly due to an effect called hot temperature stress (HTS). HTS may cause electric charges, which are accumulated in the floating gate of a programmed memory cell, to flow out of the floating gate and into the substrate or bulk layer. As the charge in the floating gate of a programmed memory cell decreases, the threshold voltage of the memory cell may become lower, as indicated by the dotted lines in FIG. 1. That is, FIG. 1 shows a range of threshold voltages corresponding to a first memory state (ST0) and a second memory state (ST1). The distance between the threshold voltages for the first and second memory states, referred to as the read margin, is shown as RM1 before the effects of HTS and RM2 after the effects of HTS. As illustrated in FIG. 1, a lowering of the threshold voltage of a programmed memory cell due to HTS may result in a reduced read margin between states (RM1>RM2). Such a reduction of read margin may reduce the reliability of a memory device and/or increase the probability of data or read errors. The effects of HTS may become even more pronounced as device feature sizes are made smaller and smaller to accommodate higher device densities.
The lowering of threshold voltage, such as lowering of threshold voltages due to HTS, can be addressed by detecting memory cells with lowered threshold voltages and reprogramming the memory cells that have lowered threshold voltages. This operation is referred to as a ‘reprogramming operation’ or ‘refresh-programming operation’. A reprogramming operation may be carried out for all the memory cells of a memory, in which case it may take a very long time to complete the reprogramming operation, during which time a reading and/or writing operation requested by a host or memory controller may be delayed. This may degrade the performance of a system employing such a flash memory.
It is known to temporarily suspend a reprogramming operation of a flash memory device using a suspending/resuming function. When a reprogramming operation of a flash memory device is suspended by a host or memory controller, a requested read or write operation is conducted, after which the interrupted reprogramming operation may resume. Such an operation may reduce the standby time of the host or memory controller. However, since the host or memory controller may provide suspend/resume commands to a flash memory device, the host or memory controller may be required to conduct complex control operations for reprogramming data in the flash memory device.